X-ray converters transform x-ray radiation into optical photons either using the indirect conversion with scintillators, by converting x-ray radiation into light or directly conversation into electron-hole pairs by using direct conversion photoconductors. The signal from the x-ray converter is sensed by a matrix of sensor elements which are typically arranged in a 2 dimensional pixel matrix or grid. Each pixel may contain a collection device for the signal in form of e.g. a photodiode to convert the optical photons into electrons, a photo gate, photo conductor or an electrode to collect the electrons or holes directly. In any case, the signal is finally stored on a capacitance in the pixel before it is read out. Resetting the storage capacitor before taking the image as well as the components involved in the readout process both contribute to electronic noise which is added to the signal. Consequently, the available signal to noise ratio is degraded.
X-ray detectors are often required to accept a large range of signals in the same image, e.g. strongly attenuated signals that have passed through dense material and direct radiation that has not gone through any material.
In order to allow correct detection of the strongest radiation, a low sensitivity setting or the low sensitivity range setting has to be chosen, with disadvantageous effect on the signal to noise ratio. In most prior art detectors, the sensitivity setting is commonly valid for all of the pixels of the detector. Mostly the amplifier or readout stage of the pixel, which is mostly formed as a source follower sampling the voltage of the capacitance of the sensor device has a limited dynamic range.
The limitation of the dynamic range forces the user to select an even lower sensitivity to avoid saturation of the readout chain.
The low sensitivity setting implies that weak radiation levels will only give small signals which are actually disturbed by the unavoidable readout noise i.e. reset noise on the storage capacitor and noise added in the readout stages.
If a higher sensitivity setting would be available in regions where only small radiation levels are present, then the signal to noise ratio would be greatly improved, hence image quality would be better.
The US Patent Application Number 2006/0231875 A1 discloses an imager with dual conversion sensitivity charge storage. A dual conversion sensitivity element (e.g., Schottky diode) is coupled between a floating diffusion region and a respective capacitor. The dual conversion sensitivity element switches in the capacitance of the capacitor, in response to charge stored at the floating diffusion region, to change the conversion sensitivity of the floating diffusion region from a first conversion sensitivity to a second conversion sensitivity. In an additional aspect, the exemplary embodiments provide an ohmic contact between the gate of a source follower transistor and the floating diffusion region which assists in the readout of the dual conversion sensitivity output signal of a pixel.
The U.S. Pat. No. 6,486,808 discloses a preamplifier stage with dynamically controllable signal sensitivity in a data signal processing circuit that includes a downstream analog-to-digital signal converter. The level of the data signal subsequent to its preamplification is monitored and the sensitivity of the preamplifier stage is dynamically adjusted in response to such data signal transcending one or more predetermined thresholds. Hence, the effective dynamic range of the preamplifier stage is extended, thereby also effectively extending the dynamic range of the overall system beyond that to which it would have otherwise been limited by the dynamic range of the analog-to-digital signal converter. Such a preamplifier is used in an x-ray imaging system such as that using flat panel solid state imaging devices.